Thermal decomposition is a serious potential accident hazard in the production and storage of thermally unstable chemicals. The characteristic reaction of a decomposing chemical evolves large quantities of heat. Under near adiabatic or well insulated conditions, the material is thus capable of sustaining a thermal runaway. The phenomenon is defined by the exponential nature of the reaction kinetics with respect to temperature. Solids, which tend to become self-insulated from their surroundings because of their low heat transfer characteristics, are particularly susceptible to incurring a thermal runaway.
Precautionary methods include the use of calorimetric anaylsis to evaluate the relative stability of reactive chemicals. The goal of the analysis is to develop an accurate knowledge of the kinetics of the decomposition reaction and heat transfer characteristics of the material. Once these parameters are known, prediction of the adiabatic or nonadiabatic thermal runaway curve is possible. This information is then used to determine safe operating and storage conditions for the material.
Generally, excellent calorimetric techniques are available for developing quantitative hazard information for predicting the behavior of thermally unstable liquids and vapors. The accelerating rate calorimeter (ARC) from Columbia Scientific Industries, Austin, Texas, is considered exemplary of the most advanced state of the prior art.
Basically the ARC consists of a contained sample which is allowed to undergo an adiabatic thermal runaway within a confines of the bomb container. Time-temperature data from this runaway is used to calculate the general kinetics describing the decomposition reaction. This information is then used for hazard prediction.
An assumption of the ARC is that the sample is a well-mixed system with good thermal contact between the sample and bomb container. Solids which characteristically have low thermal conductivity, and thus a marked tendency to heat non-uniformly, therefore, become a problem in the ARC even at low self-heat rates. Since the measuring thermocouple is located on the external surface of the bomb, the ensuing time-temperature data of the bomb thus may not accurately reflect what the sample actually underwent. Also, since the solid sample is not well mixed and has low heat transfer characteristics, the temperature throughout the solid sample itself may not be satisfactorily uniform. Imbedding the thermocouple directly in the sample, therefore, can still yield inaccurate and nonreproducible results. The same problems that occur with solid samples in the ARC also tend to occur in other prior state of the art calorimeters since similar assumptions are made. Such devices are thus generally poorly suited to adequately account for the low thermal conductivity characteristics of solid samples, hence tending to introduce errors in the data generated by the experiment.